1. Field of the Invention
This invention relates to the use of special nonionic organic dialkyl compounds for preventing fatty spew on leather.
2. Discussion of Related Art
Apart from tanning agents, oiling preparations are the most important auxiliaries for developing the character of leather. Oiling preparations develop their effect by lubricating and thus insulating the leather fibers and by hydrophobicizing the fibers. Coating the leather fibers with a fatty film reduces mutual friction and, hence, improves the suppleness and elasticity of the tissue. This has positive effects on the tear strength of leather because, in an elastic material, many fibers on exposure to tensile stress align themselves in the direction in which the stress is applied and, in doing so, offer greater resistance to tearing than the same fibers in a brittle material.
Leather oiling preparations are generally vegetable and animal oils, fats and waxes, the hydrolysis, sulfonation, oxidation and hydrogenation products obtained from these materials by chemical transformation and, finally, mineral oiling preparations; more specifically:
Saponifiable fats and oils and natural waxes and resins belong to the esters. To the leather expert, oils and fats are understood to be esters of glycerol and fatty acids which are solid or liquid at room temperature. From the group of animal fats, train oils, fish oil, beef tallow and neat's foot oil in particular are used for oiling leather; from the group of vegetable fats, castor oil, rapeseed oil and linseed oil in particular are used. In waxes and resins, the fatty acids are esterified with relatively high molecular weight alcohols instead of glycerol. Examples of waxes are beeswax, Chinese wax, carnauba wax, montan wax and wool grease; the most important resins include colophony, birch bark oil and shellac.
The chemical transformation of vegetable and animal fats gives products which are soluble in water and which, in addition, have an emulsifying effect to varying degrees on water-insoluble fats. Known products of this type are, for example, the sulfonated water-soluble oils of various kinds, train oils modified by oxidation (known as Degras or Moellon), the soaps obtained in the hydrolysis of natural fats, hydrogenated fats and, finally, free fatty acids and also stearic acid as hot-stuffing fats. Most animal and vegetable fats have a certain affinity for leather which can be considerably increased by the introduction or exposure of hydrophilic groups.
Mineral oiling preparations are also important in the manufacture of leather. These hydrocarbons are similar to natural fats and oils in some properties, but cannot be saponified. They are fractions from the distillation of petroleum which are called mineral oil in liquid form, vaseline in paste-like form and paraffin in solid form.
In many cases, however, unwanted stains are formed with time on the surface of the tanned and oiled leather. This phenomenon is known as fatty spew. Fatty spew is formed mainly on chrome-tanned leathers after relatively short or prolonged storage as a white, often bloom-like coating which covers the surface of the leather either locally or completely. The spew is attributable to the egression of solid fats from the leather. It can be caused by the natural fat basically present in the leather or by fats which have been introduced into the leather during the oiling process.
Fatty mixtures used for oiling leather tend to cause fatty spew in particular when they contain large quantities of free fatty acids. Free fatty acids generally have a higher melting point than their glycerides. The hydrolysis of fats during storage of the leather correspondingly increases the danger of fatty spew (cf. B. Kohnstein, Collegium 1913, 68; W. Fahrion, Chem. Umschau 1917, 29), although the fatty spew need not consist solely of free fatty acids. Hydroxyfatty acids can also cause fatty spew (C. Rie.beta., Collegium 1926, 419).
Soaps and fat liquors are hydrolyzed in chrome leather with release of fatty acids, especially in chrome leather which has not been sufficiently deacidified. Sulfonated oils and fats differ in their tendency to form fatty spew, the tendency to form fatty spew generally decreasing with longer life (A. Pankhurst, R. G. Mitton, R. F. Innes, N. Johnson, Journal of International Society of Leather Trades Chemists 1952, 379).
Fatty spew occurs more easily, the more fats with a tendency towards fatty spew which the leather contains. The quantity, composition and position of the fatty mixture of natural fat and fat liquor present in the leather critically determine the extent and the composition of the spew (cf. O. Grimm, Osterr. Lederzeitung 1954, 253). Leather with a loose structure is less likely to form fatty spew than leather with a dense fiber structure. Fatty spew is observed more commonly at low temperatures than at relatively warm outside temperatures.
The crystalline fatty spew develops in the hair follicles and glandular channels, small crystals initially being formed low down and gradually filling the entire hair follicle as relatively large fatty crystals, spreading over the surface of the leather and matting together to form a dense crystal film. Any fats containing stearic or palmitic acid derivatives can cause crystalline fatty spew, the danger of spew formation increasing with increasing concentration (cf. O. Hagen, Schweiz, Ledertechn. Rundsch. 1949, 1).
So-called neutral fats, i.e. substances suitable for oiling leather which do not contain any ionic groups in the molecule, for example fats, waxes and hydrocarbons, have a particular tendency to form fatty spew. Neutral fats in the form of stearic and/or palmitic derivatives, for example corresponding triglycerides, are particularly critical in this regard. Particular significance attaches to neutral fats in the case of leather chrome-tanned by the so-called fat liquor process. A fat liquor typically contains around 20 to 40% by weight of an emulsifier and 60 to 80% by weight of a neutral fat. In the leather-processing industry, it is common practice to use neutral fats which can readily be sulfated for oiling tanned leather. If, for example, a triglyceride is reacted with less than the equivalent quantity of concentrated sulfuric acid or oleum, the reaction mixture obtained may be directly used as a fat liquor because it contains on the one hand an emulsifier in the form of the sulfated triglyceride and, on the other hand, the neutral fat in the form of the unreacted triglyceride. Specific reference is made in this regard to the Examples of the present application.
In addition to the use of oiling preparations from the group of neutral fats, the natural fats already present in leather play a significant part in the formation of fatty spew.
Sebum consists of a mixture of lipids of different composition of which the type and proportions are shown in Table 1 for a few important animal species (cf. Martin Hollstein, "Bibliothek des Leders"; Vol. 4: Entfetten, Fetten und Hydrophobieren bei der nederherstellung (Defatting, Oiling and Hydrophobicizing in Leather Manufacture); pages 116-136). It is clear from Table 1, that, in quantitative terms, the triglycerides are the most important group of skin lipids. From the point of view of the leather expert, a particularly critical factor in connection with the formation of fatty spew is that these triglycerides are based for the most part on saturated fatty acids: lauric acid, myristic acid, palmitic acid and stearic acid are typical units within this group. The wax esters also contain inter alia these particularly critical fatty acids as units.
TABLE 1 ______________________________________ Composition of the total lipids of the skin of various animal species (in %, based on total lipid of the fresh skin) Animal species Lipid Cow Goat Sheep ______________________________________ Triglycerides 53 68 56 Wax esters 11 12 23 Phospholipids 1 8 6 Cholesterol 4 8 5 Free fatty acids 0 4 5 Hydrocarbons 1 -- -- ______________________________________
Accordingly, it is clear that the natural fat already present in leather represents a constant latent potential for the formation of fatty spew. In addition, it is known that the natural fat content of the raw material has remained consistently high over the past 10 years. According to Ernst Pfleiderer, the reason for this lies in the changing methods used to breed and feed cattle. The leather-processing industry has complained for years about the increased fat content in major skin provenances, calf skins and pig skins (cf. Das Leder, 1983 [34] 181-185).
Pfleiderer's comments were confirmed in our own exploratory studies on limed and skived cowhides. Natural fat contents of up to 8% and higher were found in the belly parts.
Accordingly, leather with a high natural fat content requires special measures to suppress its tendency to form fatty spew. For example, it is possible and quite common in practice to defat the leathers accordingly, although this does involve a special process step. Other possibilities are almost meaningless for practical purposes.
Fatty spew, which can be clearly distinguished from mineral spew by its disappearance on heating with a burning match, can be removed for example by rubbing the leather with a cloth soaked in spirit. To prevent the subsequent reformation of spew, it has been recommended to oil the grain with a neutral mineral oil (cf. F. Stather, "Gerbereichemie und Gerbereitechnologie", Berlin 1967, page 740). A. Gluszcak and K. J. Bienkiewicz report on the use of a mixture of wood dust, water, hexane and tetrachloromethane for removing fatty spew (cf. Przegl. Skorzany 1985, 40(11-12), 232; reported in Chem. Abstracts 105(6):45160e).
The formation of fatty spew on commercial clothing and glove leather observed over a period of 2 to 4 weeks could be prevented by the use of glutaraldehyde in the tanning process or by using a fat liquor containing mineral oil (cf. A. Gluszcak, K. J. Bienkiewicz, Przegl. Skorzany 1985, 40(11-12), 232; reported in Chem. Abstracts 105(6):45160e). However, Gluszcak and Bienkiewicz's method has the disadvantage that it is confined to a special tanning method. However, it does not apply to chrome-tanned leathers which still represent by far the highest percentage of all commercial leathers.
However, since oiling is in any event an almost essential step after tanning in the processing of leather in order to achieve the required product properties, it has become common practice to use special synthetic oiling preparations with only a minimal tendency to form fatty spew.
A class of oils which have been widely used for this purpose are halogenated compounds, such as chlorinated hydrocarbons. Unfortunately, the increasingly more stringent ecological and toxicological requirements which products entering the environment or coming into contact with the consumer are expected to satisfy make this class of compounds increasingly unattractive. The use of chlorinated paraffins as additives for fat liquor emulsions to prevent the formation of fatty spew on chrome-tanned pig skin is described, for example, by J. Golonka (Przegl. Skorzany 42(2), 35; reported in Chem. Abstracts 107(18):156865z).
On the whole, therefore, the methods for preventing fatty spew known from the prior art are unsatisfactory.
It is clear from the foregoing context that there is a continuing need in the leather industry for additives and oiling preparations which effectively prevent fatty spew in order thus to extend the range of commercial products and to be able to respond flexibly to changing market requirements. Above all, there is a need for ecologically and toxicologically safe additives and oiling preparations which, in their practical application, do not lead to the unwanted formation of fatty spew.